The announcement, which had been expected, is seen as an attempt to force Congress to deal with the Postal Service’s increasing financial woes. Congress has tried to reorganize the agency, but efforts have been derailed because of politics.

Material prepared for a Wednesday news conference by Postmaster General Patrick R. Donahoe says that Postal Service market research and other research has indicated that nearly 7 in 10 Americans support the switch to five-day delivery as a way for the agency to reduce costs.

“The Postal Service is advancing an important new approach to delivery that reflects the strong growth of our package business and responds to the financial realities resulting from America’s changing mailing habits,” Donahoe said. “We developed this approach by working with our customers to understand their delivery needs and by identifying creative ways to generate significant cost savings.”

The Postal Service contends that it has the authority to cut back service, though some in Congress insist that lawmakers have the final word. In any case, the announcement is expected to move the issue to a front burner.

According to postal officials, the end of Saturday delivery would begin on Aug. 1. The service said it will continue to deliver packages on Saturdays.

Ending Saturday service is estimated to save the agency about $2 billion a year. Officials said it lost about $16 billion in fiscal 2012, which ended Sept. 30 — about three times the loss it had a year ago.

The proposal comes as the agency has closed post offices and changed procedures to try to stem its hemorrhaging of billions of dollars in recent years. More people have been turning to alternatives to traditional mail service, including electronic tools such as email and outside competitors such as FedEx, both of which have seriously cut into the Postal Service’s erstwhile monopoly.

Congress in recent years has prohibited the Postal Service from dropping Saturday mail delivery, but the initial response to Wednesday’s announcement was more accepting of what might be inevitable.

“This common-sense reform would save the Postal Service more than $2 billion annually,” two top Republicans, Rep. Darrell Issa of California and Sen. Tom Coburn of Oklahoma, said in a statement.

“In his recent inaugural address, President Obama spoke about the need to find real solutions to our nation’s problems. Supporting the U.S. Postal Service’s plan to move forward with 5-day mail delivery is one such solution worthy of bipartisan support,” they said.

Rep. Elijah E. Cummings of Maryland, the top Democrat on the House Oversight and Government Reform Committee chaired by Issa, agreed that action was needed, but questioned whether the Postal Service could act alone.

“The Postal Service’s declining mail volume poses a significant challenge, and the enactment of comprehensive postal reform legislation must be an urgent priority for the current Congress,” Cummings said in a statement. “However, the issue of service delivery frequency should be addressed in that legislation rather than through arbitrary action by the Postal Service.”

Recent national polls show that a majority of respondents support ending Saturday mail delivery, and Obama has proposed halting such service as part of his budget-cutting proposals. Though the Postal Service is a quasi-governmental, self-funding entity, its worker compensation and retirement plans are tied to the federal budget.

Lawmakers have tried unsuccessfully for years to overhaul the Postal Service, but efforts have fallen victim to different needs in different areas. Representatives of rural areas tend to argue that traditional mail delivery is needed, a position backed by magazine publishers and other heavy users of the Postal Service.

The president of the National Association of Letter Carriers, Fredric Rolando, said in a statement that the end of Saturday mail delivery is “a disastrous idea that would have a profoundly negative effect on the Postal Service and on millions of customers,” particularly businesses, rural communities, the elderly, the disabled and others who depend on Saturday delivery for commerce and communication.